From helical to planar chirality by on-surface chemistry

Abstract

The chirality of molecular structures is paramount in many phenomena, including enantioselective reactions, molecular self-assembly, biological processes and light or electron-spin polarization. Flat prochiral molecules, which are achiral in the gas phase or solution, can exhibit adsorption-induced chirality when deposited on surfaces. The whole array of such molecular adsorbates is naturally racemic as spontaneous global mirror-symmetry breaking is disfavoured. Here we demonstrate a chemical method of obtaining flat prochiral molecules adsorbed on the solid achiral surface in such a way that a specific adsorbate handedness globally dominates. An optically pure helical precursor is flattened in a cascade of on-surface reactions, which enables chirality transfer. The individual reaction products are identified by high-resolution scanning-probe microscopy. The ultimate formation of globally non-racemic assemblies of flat molecules through stereocontrolled on-surface synthesis allows for chirality to be expressed in as yet unexplored types of organic-inorganic chiral interfaces.

Strain-induced skeletal rearrangement of a polycyclic aromatic hydrocarbon on a copper surface

“…C–C bonds at the molecular edges are seemingly emphasized because the long-range attractive forces that appear as blurred depressions in the images contribute predominantly at the molecular centre32. It is well known that a frequency-shift (Δ f ) contrast depends strictly on the atom height with respect to the surface; if a molecule is not flat but twisted or bent, the Δ f signals in the sterically protruded region in the molecule must be much larger (that is, more repulsive against the probe tip)314146535455. In the images of DAPh and DAA, on the other hand, all covalent bonds within the molecules are clearly resolved, suggesting that both of the intrinsically distorted PAHs adsorbed flatly onto the surface.…”

“…Adsorbate–substrate interactions can promote characteristic thermal reactions (namely, on-surface synthesis2122) occurring in neither the crystal nor the solution, which can yield well-defined products such as graphene nanoribbons23242526 and other nanocarbons2728293031. Therefore, we conceive that the geometric structure of a PAH can be distorted by adsorption onto a metal surface, so that the molecule may store strain energy to facilitate a catalytic reaction at the surface.…”

“…Therefore, we conceive that the geometric structure of a PAH can be distorted by adsorption onto a metal surface, so that the molecule may store strain energy to facilitate a catalytic reaction at the surface. Although some previous studies have implied that the structural distortion of precursors on surfaces is a key factor for on-surface reactions24262831, the correlation between the structural strain and reactivity has not been focused on in the previous studies and remains less well understood. Besides, the binding of a target molecule to a flat surface provides an overwhelming advantage in the characterization of individual molecules on the surface with scanning tunnelling microscopy (STM) and atomic force microscopy (AFM).…”

“…In particular, noncontact AFM with a functionalized tip (that is, a probe attaching an atom or a molecule to its apex) can visualize chemical bonding structures of adosorbates323334353637, because it can sensitively detect the repulsive forces between atoms in the adsorbate and the sharpened tip apex3839. By using this imaging technique, reactants, intermediates and products of various on-surface reactions have successfully been identified3140414243444546. We focus on the on-surface reactivity of azulene-derivative PAHs.…”

On‐Surface Bottom‐Up Synthesis of Azine Derivatives Displaying Strong Acceptor Behavior

“…These surface‐stabilized species can lead to compounds that are difficult or impossible to obtain via conventional synthetic procedures 9, 10, 11, 12…”